This invention relates to video imaging devices used to capture images of objects on a sequential basis. More particularly, this invention relates to triggered image capturing devices using a camera with solid state video imaging technology.
Solid state video imaging devices are known which are designed to capture still images of objects in a time sequential manner. Such devices are usually triggered either internally or externally by means of trigger pulses, which are either generated on a regular periodic basis or in response to the external generation of a trigger pulse in response to the passage of an object past a triggering point. Typical uses of such devices are in industrial applications in which it is desired to capture static images of moving objects along an assembly line, a conveyor belt or the like.
In known systems, a video camera using a charge coupled device (CCD) imaging element is installed at an observation point and is triggered either on a regular timed interval or in response to a triggering signal generated by a monitoring device (such as a photoelectric sensor) whenever an object is properly positioned in the observation area. In response to the trigger signal, an electronic image is captured by the CCD in the form of stored charges which are then converted into appropriate analog video signals. These video signals are then processed by follow on signal processing circuitry external to the camera and subsequently coupled to appropriate display and storage devices, such as a video monitor, a video tape recorder or the like.
While effective in many applications, known CCD based systems suffer from limited image resolution due to two factors. The first factor is inherent in the construction of conventional CCD video cameras with CCD elements having a lateral overflow drain structure. At the beginning of each imaging cycle, any charges accumulated in each pixel region of the CCD imaging element must initially be discharged prior to beginning development of the desired image (to remove background noise). With CCD devices having lateral overflow drain structure, the duration of the minimum discharge period is such that the practical shutter speed is limited to about 1/2000th second. For application requiring a faster shutter speed, such as objects traveling along a belt conveyor at a rapid speed or on-line inspection installations, the required shutter speed ranges from a minimum of 1/4000th second to 1/30000th second in order to provide a static image of the moving object with sufficient resolution. The second factor lies in the normal format arrangement employed in conventional CCD video cameras: such cameras typically produce video in an interlaced format in which each frame is divided into two successive fields, as in the standard NTSC video signal format. With this type of video formatting, the resolution obtained for each image is only that of one field, which is approximately one-half the resolution of a full frame. Thus, even if the initial discharge period of such devices were somehow decreased by an amount sufficient to permit shutter speeds in the required range, the resolution of the image obtained would not meet the quality standards required for many applications.
Efforts to date to design electronically shuttered solid state video imaging devices devoid of the above disadvantages have not met with success to date.